Thermal imaging cameras

ABSTRACT

A modular hyperspectral thermal camera that combines a wide field-of-view with a low erroneous recognition rate is described. The modular hyperspectral thermal camera provides such low erroneous recognition rates without any requirement for cryogenically cooling the associated optical components. The modular nature of the hyperspectral thermal camera permits easy exchange of the optical components and so provides a device that is easily calibrated and varied in resolution. In addition the modular nature allows the hyperspectral thermal camera to be readily converted to a broad band thermal camera, a full field spectrograph or a thermal bandpass filter camera, as required.

This application is a continuation of U.S. application Ser. No.10/470,702 filed Feb. 27, 2004 now U.S. Pat. No. 7,282,715 which claimspriority to PCT/GB02/00397 filed Jan. 30, 2002, which claims priority toGB Patent No. 0102529.5 Filed Jan. 31, 2001, the contents of which arehereby incorporated by reference.

BACKGROUND

The present invention relates to the field of thermal imaging camerasand in particular to improvements of such cameras for detecting mediumwave infrared and long wave infrared regions of the electromagneticspectrum.

A principal application for thermal imaging cameras is the detection,recognition and subsequent identification (DRI) of objects. Presentcameras are required to render to a display screen or to an “imageprocessor” the “shape and texture” attributes of such objects and theircontexts to such a quality that a human observer or an electronicsubstitute may perform these tasks to a high probability of success. Theresolution of such devices is limited to the ability of humans, or theirelectronic substitute, to recognise objects from the rendering on adisplay screen.

When combined with the achievable performances of cameras and humanobservers and processors, these requirements frequently impose limits onthe camera's maximum field of view to such an extent that the ranges atwhich the tasks of DRI can be achieved are incompatible with manyapplications of the camera. Within the limits of technology and thoseimposed by natural laws, an increase in the “task achievement range”requires a reduction in the field of view of the camera. With thisnarrowing field-of-view, the probability of an object being present inthe field is reduced. Furthermore, any decrease in the field of view ofthe camera is likely to result in an increase in the area of the opticalaperture with consequent impact on the cost and vulnerability of theoptics and the aerodynamic performance of any aircraft on which thecamera is deployed.

If the intended application requires a minimum field-of-view, then theability of the camera to recognise objects is adversely affected and thecamera has only sufficient resolving power to detect objects. Such acamera is then limited in its ability to discriminate between objectsbecause the context will inevitably contain multiple features such asanimals, heated rocks or vegetation that have the same temperaturedifference as that created by the genuine object. In such a situation,the application of the camera is limited by erroneous recognition.

The prior art teaches of thermal cameras characterised by a widefield-of-view and a low erroneous recognition rate. Such devices areemployed for the measurements of the spectral emissivity of natural andcultural objects in the so-called Medium Waved Infrared (MWIR), between3.2 um and 5.5 pm, and Long Wave Infrared (LWIR), 7.8, um and 11.4 um,atmospheric windows. It is known to those skilled in the art that theuse of such a camera capable of measuring these attributes enhances theobserver's ability to discriminate between classes of object such astrees, rocks, grasses and vehicles.

A thermal imaging camera with such a capability is known as ahyperspectral camera. Rather than observing the scene using a singlewaveband and presenting the image as a plane, the scene is decomposedinto a number of planes representing spectral sub-bands or spectralbins. The assembly of these planes is then known as a “hyperspectralcube”.

It is well known to those skilled in the art and science that suchhyperspectral cameras present difficulties in achieving adequate signalto noise ratio (SNR) against objects of interest whose temperaturedifference relative to the background is typically only a few Celsius.In a perfect thermal imaging camera, the noise in the instrument isdominated by that from the detector. To achieve such a performance, thenoise internal to the detector itself must be made extremely low. Thiscan only be achieved in detectors sensitive to LWIR radiation bycryogenically cooling the detector. Modern detectors are integrated witha closed-cycle cooling engine which can reduce the temperature of thedetector array to values lower than 80 Kelvins. When fitted with such adetector, the camera is then capable of achieving “BackgroundLimited”thermal sensitivity. This performance level indicates that thenoise in the camera is created by the random arrival of photons from allobjects in the field-of-view of the detector. The photon rate, and thefluctuation thereof, are determined by the temperature of the objects.As that temperature falls, so does the noise level in the detector.

This effect is exploited in modern, high performance, infrared detectorsby engineering the detector package and cooling engine to cool not onlythe detector array but also a “cold-shield” enclosing the detectorarray.

The cold-shield is pierced to allow the detector to receive the sceneimage-forming rays from the imaging system such as a sequence of lensesor mirrors.

Inconsiderate design of this optical system leads to an instrument whosedetector is exposed not only to radiation from the scene but also tothat from the interior of the camera. Contributions to this additionalradiation come either from the optical elements or from the enclosure,either directly or by reflections thereof from the optical components.

If the camera design is such that spectral filtering is provided priorto this process of intrusion by stray radiation, the SNR of theinstrument will be adversely affected and will not achieve that possibleif both the signal and noise had been spectrally filtered.

Prior designs of hyperspectral thermal cameras have solved this problemin a number of ways. A choice between the various methods is mainlyinfluenced by the requirements of spectral resolving power and theoperating waveband. The ratio of the operating waveband to the spectralresolving power is described by the term “number of channels” or “numberof spectral bins”.

For a camera with only a modest number of spectral bins, a preferredmethod is to introduce a carousel of dielectric interference filters atthe entrance window of the detector. Rotation of the carousel allowsmeasurements of the radiation transmitted through the filter. Theadvantage of this method is that out-of band radiation is reflected fromthe filter out to the optical system and either absorbed in the camerabody or reflected out of the camera. Thus, the noise from the cameraoptics is also filtered. Another advantage of this method is that a fullspatial frame is gathered during the dwell time of the filter. Thedisadvantage of this method is that the behaviour of interferencefilters is very dependent upon the angle of arrival of rays.

Thus when used with focusing optics, the spectral bandpass of the filteris widened and the number of spectral bins is limited to less than about8 in the LWIR band.

Higher spectral resolving power can be achieved by using a spectrallydispersive component such as a prism or a diffraction grating. Theprincipal disadvantage of a prism instrument is that the dispersivepower of prisms is relatively low so that long focal lengths and thusbulky imaging optics are required to form a usefully sized spectrum. Inaddition, light from the interior of the camera is uncontrolled and willincrease the noise.

Thus, it is normal for such instruments optical components to be cooledto a very low temperature such that this intrusive radiation is reduced.In the very highest quality instruments it is normal to cool the entireinstrument which may weigh 100 kg with a cryogenic liquid such ashelium. This cooling requirement eliminates such instruments fromlarge-scale deployment that requires maneuverability. The reflectivediffraction grating has a very high dispersive power and is widely usedin laboratory instruments, but these are also bulky. The obliqueconfiguration of the instruments using reflection diffractive gratingsalso limits their use to optics with relatively poor light-gatheringcapacity and field-of-view at which high image quality is possible.

The highest spectral resolving power is achieved with an instrumentusing a variable optical path interferometer.

This capability is gained at the penalty of poor light gatheringcapacity and extreme sensitivity to relative mechanical motions of thecamera components.

SUMMARY OF THE INVENTION

It is an object of the present invention is to provide a hyperspectralthermal camera that combines a wide field-of-view with a low erroneousrecognition rate.

It is a further object of the present invention to provide ahyperspectral thermal camera that provides a low erroneous recognitionrate without any requirement for cryogenically cooling the associatedoptical components.

It is yet a further object of the present invention to provide ahyperspectral thermal camera that employs a modular optical system thatpermits easy exchange of optical components. The easy exchange ofoptical components provides a device that is easily calibrated, variedin resolution, and that can be readily converted from a hyperspectralthermal camera to a broad band thermal camera, a full field spectrographor a thermal bandpass filter camera.

According to the present invention there is provided a modularhyperspectral thermal imaging camera comprising a transmissivediffraction grating, a detector and optical components, wherein theoptical components of the modular hyperspectral thermal imaging camerado not required to be cryogenically cooled.

Preferably the transmissive diffraction grating comprises a linear phasegrating and a refractive substrate, characterised in that for radiationof a predetermined wavelength the induced diffraction of the linearphase grating compensates the induced refraction of the refractivesubstrate, such that the reference radiation passes undeviated throughthe transmissive diffraction grating.

Preferably the detector comprises an aperture stop, a cold shield and aphotodetector array.

Preferably the aperture stop is formed by piercing the cold shield.

Alternatively the aperture stop is formed by piercing a convex mirrorsituated externally to the cold shield, such that the radius ofcurvature of the mirror is equal to its distance from the photodetectorarray.

Preferably the photodetector array comprises a mosaic of photodiodes.

Preferably the optical components of the hyperspectral thermal imagingcamera comprises an entrance slit, an imaging lens, a collimator and afocusing lens.

Preferably the optical components are formed from materials that exhibitvery low absorption coefficients.

Preferably the entrance slit is formed by a transparent piercing to air,in a highly reflective surround.

Alternatively the entrance slit is formed by a transparent piercing to atransmissive material, in a highly reflective surround.

Optionally the entrance slit surround is heated to a temperature justabove the dew point of the atmosphere.

Preferably the entrance slit is located internally to the imaging lenswhereby the imaging lens comprising a singlet and an air spaced doublet.

Alternatively the entrance slit is located externally to the image lenswhereby the imaging lens comprises an air spaced doublet and a singlet.

Most preferably the imaging lens is designed such that the image raysare telecentric.

Optionally, a pair of mirrors are inserted allowing the detector to beilluminated by a pair of reference sources chosen to be at knownoperating temperatures whereby the device is calibrated.

Optionally a scanning mirror may be inserted after the entrance slitallowing the field of view of the slit to be scanned through the objectfield.

Preferably the collimator comprising a negative aspheric lens and apositive aspheric and diffractive hybrid lens.

Most preferably the collimator is a focal.

Preferably the focusing lens comprises a positive lens and a correctinglens. Most preferably the focussing lens is designed such that the imagerays are telecentric thereby forming an image on the detector.

Optionally, a cooling jacket for the optical components is employed toenhance the signal to noise ratio of the detected image.

Most preferably all the components of the hyperspectral thermal imagingcamera are easily interchangeable.

Preferably the resolution of the device may be altered by changing thetransmissive diffraction grating.

According to a second aspect of the present invention there is provideda method of calibrating a modular hyperspectral thermal imaging cameracomprising: 1. Inserting one or more reference mirrors within themodular hyperspectral thermal imaging camera; and 2. Illuminating adetector with one or more reference sources of known opera tingtemperatures.

According to a third aspect of the present invention there is provided amethod of converting a modular hyperspectral thermal imaging camera to abroad band thermal imaging camera comprising the removal of atransmissive diffraction grating associated with the modularhyperspectral thermal imaging camera.

According to a fourth aspect of the present invention there is provideda method of converting a modular hyperspectral thermal imaging camera toa full field spectrograph comprising: 1. Removing an entrance slit; and2. Rotating a transmissive diffraction grating; associated with themodular hyperspectral thermal imaging camera.

According to a fifth aspect of the present invention there is provided amethod of converting a modular hyperspectral thermal imaging camera to athermal bandpass filter camera comprising the insertion of a bandpassfilter prior to the detector associated with the modular hyperspectralthermal imaging camera.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, with reference to the accompanying drawings, in which:

FIG. 1 illustrates a schematic presentation of a hyperspectral thermalimaging camera;

FIG. 2 illustrates a schematic presentation of a grism of thehyperspectral thermal imaging camera of FIG. 1;

FIG. 3 illustrates a schematic presentation of an alternative focussinglens of the hyperspectral thermal imaging camera of FIG. 1; and

FIG. 4 illustrates an alternative embodiment of the hyperspectralthermal imaging camera of FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring initially to FIG. 1 a hyperspectral thermal imaging camera 1can be seen to comprise of an imaging lens 2, an entrance slit 3, anafocal collimator 4, a grism 5, a focusing lens 6, an aperture stop 7and a detector 8.

The imaging lens 2 comprises a sequence of three individual lenselements, namely a Gallium Arsenide (GaAs) lens 9, a Zinc Selenide(ZnSe) lens 10 and a Thallium Bromo Iodide lens (KRS-5) 11. Theseindividual lenses are arranged so as to form an imaging lens whoseoptical performance in the infrared waveband, 8 to 11 m, is essentiallylimited by diffraction at the aperture stop 7.

The materials chosen for the imaging lens 2 are characterised in thatthey exhibit low to negligible absorption, and thus emission, ofradiation in the operating waveband. Furthermore the performance of therefractive index properties of these lenses are substantiallyindependent of variations in temperature.

The design of the imaging lens 2 is such that the image rays aretelecentric with the majority of the energy contained within the slitwidth over the full wavelength band and over the full image height. Aresult of these properties is that it is not necessary to provide afocus control to ensure that the image of the object scene remainsperfectly focussed at the detector 8.

The entrance slit 3 is formed by a transparent piercing in a highlyreflective surround. In this embodiment the piercing is to air however,it could be to a transmissive material with or without a curved surfacee. g. a lens component of the imaging lens 2. The use of a conductingtransparent material such as Gallium Arsenide allows the slit surroundto be heated to a temperature just above that of the dew point local tothe hyperspectral thermal imaging camera 1. Thus the critical opticsunits may be cooled substantially without atmospheric water condensingat the entrance slit 3.

Employing Gallium Arsenide for the entrance slit surround has a furtheradvantage in that it greatly reduces the amount of radiation energy fromthe slit surround tailing on the detector 8. This has the effect ofavoiding an increase in the noise in the signal received at the detector8.

The a focal collimator 4 comprises a negative Zinc Selenide lens 12 anda positive Gallium Arsenide lens 13.

The negative lens 12 is aspheric while the positive lens 13 is anaspheric and a diffractive hybrid lens. This combination of a negative12 and positive lens 13 provides an afocal collimator 4 that exhibitstelecentric properties.

FIG. 2 shows a schematic representation of the grism 5.

This is an optical element comprising a linear phase grating 14 cut onthe surface of a refractive prism 15. It is characterised in that only apredetermined reference wavelength passes undeviated through the grism5. The undeviated wavelength is that which is twice the optical stepheight of the linear phase grating 14, where the optical step height isa function of the geometrical height of the step and the refractiveindex of the substrate refractive prism 15. The spacing between thesteps of the linear phase grating 14 determines the angle through whichan incoming wavefront is diffracted.

The focusing lens 6 comprises a Gallium Arsenide positive lens 16 and aZinc Selenide corrector 17. The focusing lens 6 is designed such thatthe image is telecentric with respect to the detector 8.

The detector 8 comprises a cryogenic cold shield 18 and a photodetectorarray 19. The hyperspectral thermal imaging camera 1 is designed suchthat the photodetector array 19 is situated at the focal plane of thedevice.

The aperture stop 7 is formed by piercing the cold shield 18.

The photodetector array 19 is made up of a mosaic of photodiodes. Thephotosensitive material is Cadmium Mercury Telluride cooled by a closedcycle thermodynamic engine to a temperature of around 70 Kelvin. Thesignals from the array are then stored in capacitors (not shown)connected to a silicon multiplexer (not shown) whose outputs arearranged to display a visible reconstruction of the thermal radiationfrom the scene pixels.

This hyperspectral thermal imaging camera 1 exploits the properties ofthe grism 5 such that it is able to filter out background noise withoutthe need for cryogenic cooling of the major optical components, as isthe case with the prior art. Such cooling is still required to beemployed at the photodetector array 19. Incident radiation is focused bythe imaging lens 2 onto the entrance slit 3. This incident radiation isthen afocally imaged by the afocal collimator before being diffracted,and hence resolved, into spectral components by the grism 5. Thefocusing lens 6 then gathers the diffracted radiation and focuses it atthe photodetector array 19. Thus, a chromatic image of the radiation atthe entrance slit 3 appears at the photodetector array 19 where it canbe read or subsequently displayed or processed.

The distribution of power and aberration through the lenses within thehyperspectral thermal imaging camera 1 is arranged such that theprincipal and marginal rays directed from the entrance slit 3 towardsthe detector 8 are sensibly normal to the first surfaces of theintervening lens elements. This arrangement minimises the visibility ofthe enclosure of the hyperspectral thermal imaging camera 1 viareflections in the lens surfaces.

The hyperspectral thermal imaging camera 1 not only images directly theradiation passing through the entrance slit 3 but also the radiationfrom the entrance slit surround. However, the careful design of opticswithin the hyperspectral thermal imaging camera 1 are such that theintensity of stray radiation incident on the photodetector array 19, andthus the background noise, is reduced to a level that is less than 10%of that which would be received from a black body at the temperature ofthe enclosure.

It is possible to reduce this background noise level still further byhousing the hyperspectral thermal imaging camera 1 in a cooling jacket(not shown). Such a cooling jacket requires only modest cooling in orderto improve the signal to noise ratio of the device, thus still avoidingthe need for further cryogenic cooling.

The modular design of the hyperspectral thermal imaging camera 1 permitsthe quick and easy interchange of the components. For example the grism5 may be easily substituted by another exhibiting either lower or higherspectral resolution at the focal plane. Alternatively, it is possible toremove the grism 5 entirely allowing the device to act as a broad bandthermal imaging camera.

The image quality of the imaging lens 2 and the afocal collimator 4 aresuch that with the entrance slit 3 removed and by rotating the grism 5about its optical axis the device operates as a full field spectrograph.A chromatic spectral cube of the two dimensional scene is obtained onthe detector array 19 such that appropriate electronic processingprovides a reconstruction of the scene spectral planes.

Replacing the imaging lens 2 with an alternative embodiment imaging lens20, as shown in FIG. 3, makes it possible to arrange the components suchthat the entrance slit 3 is external to the imaging lens 20. The imaginglens 20 is a Petzval type arrangement comprising an air spaced doublet21, formed from a Gallium Arsenide (GaAs) lens 22 and a Zinc Selenide(ZnSe) lens 23 and Thallium Bromo Iodide (KRS-5) singlet 24. Theairspace between the imaging lens 20 and the afocal collimator 4 is suchthat switch mirrors (not shown) may be inserted to allow easycalibration of the hyperspectral thermal imaging camera 1.

With this optical configuration the hyperspectral thermal imaging camera1 is such that the entrance slit 3 is substantially ahead of the firstoptical element. As a result a scanning mirror (not shown) may be easilyinserted so allowing the field of view of the slit through the objectfield to be scanned.

An alternative embodiment of the hyperspectral thermal imaging camera isshown in FIG. 4. In this embodiment a bandpass filter 25 is located justprior to the detector 8. The aperture stop is now formed by piercing ahighly reflective mirror substrate 26 whose mirrored side is sphericallycentred at the centre of the detector array. In this embodiment thereflective mirror substrate 26, and hence the aperture stop, is externalto the cryogenic detector enclosure.

Positioning of bandpass filters 25 in a carousel wheel (not shown)allows the spectral pass band to be selected.

Therefore in this embodiment the hyperspectral thermal imaging cameraoperates as a thermal bandpass filter camera that does not requitecryogenic cooling to achieve an efficient signal to noise ratio.

The design of the hyperspectral thermal imaging camera has the advantagethat it removes the need to cryogenically cool the optical components inorder to achieve a workable signal to noise ratio.

It is a further advantage of the invention that its modular natureallows its components to be easily exchanged. Therefore the inventioncan be easily altered between a hyperspectral thermal imaging camera, abroad band thermal camera, a full field thermal spectrograph, or abandpass filter thermal camera.

A further advantage of the invention is that is applicable to both theMWIR and the LWIR wavebands using the same materials.

A yet further advantage of the invention is that the overall angularresolution or spectral resolving power of the camera may be changed byreplacement of the grism.

Further advantages of the present invention are that the opticsnaturally provide a means for internal calibration and compensation forthe temperature “gain” and “offset” errors that are unavoidable withMWIR and LWIR detectors.

Further modifications and improvements may be added without departingfrom the scope of the invention herein intended.

1. A method of converting a modular hyperspectral thermal imaging camerato a full field spectrograph, comprising: removing an entrance slit; androtating a transmissive diffraction grating associated with the modularhyperspectral thermal imaging camera.